The present invention relates generally to digital communications. More particularly, the invention relates to an asynchronous communications system which employs a three or four signal serial communications scheme for providing bidirectional transfer of data between master and slave devices.
The proliferation of digital computer equipment and peripheral equipment has given rise to a wide variety of different and incompatible communications schemes. There are the so-called RS-232 and RS-422 standards for interconnecting peripheral devices and data collection devices to computer systems. However, these schemes are far from being standards since there can be many different variations in pinout, data formats, baud rates and so forth. For example, in order to have the proper pinout to establish communication over an RS-232 line, it is necessary to know which of the data terminal equipment is originating the communication and which is receiving the communication. Furthermore, it is necessary to know the communication protocol, the number of start bits, stop bits and data bits, whether error checking is implemented, and if so whether even parity or odd parity is being used. Finally, it is also necessary to know the baud rate and whether the devices implement some form of hardware handshaking via other control lines. All of these factors add to the complexity of a communications system, often making it difficult for the end user to connect peripheral equipment to a computer system and make it work.
The present invention overcomes all of these difficulties with prior art communications schemes, making it easy for the end user to interconnect computer equipment and peripherals. The communications system of the invention eliminates the need for matching baud rates and data formats between devices. The invention enhances error detection methods to allow a transmitting device to correct errors. It eliminates timing circuits that are required by other communications schemes to synchronize bit patterns. It reduces the hardware to a minimum without hindering the performance of the communicating devices. It provides software features that can be universally applied to all communicating devices and allows for expandability of hardware and software for all applications, while maintaining compatibility at all levels of expansion. The invention maintains high speeds of communication and minimizes power requirements, cost and space of both hardware and software.
In accordance with the invention, the communications system comprises a master, a slave and a communication transmission means connecting the master and slave. The communication means, which may be a multiple wire transmission line or an electromagnetic or optical communication link, has a master line, a slave line and a call line. The master is adapted to selectively place the master line in high and low logical states. Similarly, the slave is adapted to selectively place the slave line in high and low logical states. The master and slave have protocol establishing means for enabling the master to communicate with the call line when the master line and the slave line are at the same logical state. The protocol establishing means enables the slave to communicate with the call line when the master line and the slave line are at different logical states.
For a more complete understanding of the invention and its many objects and advantages, reference may be had to the following specification and to the accompanying drawings.